HD Video cable - drops video / audio

I just bought the HD cable. It seems to drop as soon as I shift the position of the Surface. I doesn't seem as though the cables are loose anywhere. I've tried locking the screen orientation before plugging the cable in, but I still get the same results. I can start up the TV display and watch use the Surface for a few minutes and start watching a video, but as soon as I tilt the Surface or lay it down, the tablet will beep and I'll lose the video, audio or both. Anyone have similar problems?

I've tried two HD TV's (both Panasonic) and three HDMI cables, all unmarked brands. All of the cables work with their usually connected DVR, Roku and TiVo devices. The cables are all 6 footers. I'll try returning the cable to the MS store, or go there and see if they can try my Surface with a setup in the store.

Yeah, I also did notice this weekend, that running a 720p-screen isn't going to work: The modern-ui needs at least a resolution of 1366x768 px to operate. With 720p you're just able to show the desktop and ... err ... run office or play-to.
In my case working means: no problems with a 1080p-tv.
Sry for these bad news.

Ps. I think, there was a post (somewhere at the internet)about changing some reg-entry to operate win8 with a lower resolution - maybe this could help.